1.) install GnuGPG on your machine. basically just run the installer, fire and forget

click to go and download GnuPG

2.) add the bundle to mail. This installer does it with an applescript

click to go and download GPGMail

Now you have gpg enabled on your machine and Mail supporting it:

new preferences pane	new options in compose window

3.) GPG Keychain Access allows you to generate your private keys (a 1 off thing) and manage people’s public keys

click to go and download GPG Keychain Access

When you first load GPG Keychain Access it’ll tell you you don’t have any keys “Would you like to generate one?”. You basically create a key (4096 bit) that never expires (0 years) for whatever email address you want to authenticate then you’ll get a header whenever you compose a message where you can optionally sign, and/or encrypt your message (encryption only works when both parties have each other’s keys - again a 1-off thing).